High Performance Titanium Dioxide And Sodium Metal Anode For Sodium Ion Batteries

In recent decades,due to the development of battery technology,lithium ion batteries have been widely used in our daily life.However,the available lithium resources can't satisfy the urgent need for establishing a large-scale high-energy-density energy storage system.Due to the abundant sodium resources,low cost,and similar operating principles to lithium ion batteries,sodium ion batteries have gradually become a substitute for lithium ion batteries in recent years.Among all kinds of anode materials for sodium ion batteries,titanium dioxide and sodium metal have broad development prospects due to their low cost and other advantages.The interface of titanium dioxide and sodium metal anode has been constructed to regulate the diffusion,insertion/extraction,nucleation and growth behavior of sodium ions in the electrode materials,which improved the sodium storage performances successfully.And the experimental results and theoretical simulations are mutually verified.The main research work of this thesis is listed as follows:(1)Carbon-modified flower-like titanium dioxide(TiO2(A)/TiO2(B)/C)with ultrathin nanosheets structure was successfully prepared through a simple solvothermal reaction with subsequently high-temperature calcination.Theoretical calculations and experimental results prove that the two-phase interface between TiO2(A)and TiO2(B)can not only provide additional active sites for reversible sodium storage,but also effectively reduce the diffusion barrier of sodium ions and promote charge transfer dynamics.At a current density of 10 C,it has a reversible capacity of 120 mA h g-1 after 6000 cycles,and a flower-like structure composed of ultra-thin nanosheets can be easily observed after 1200 cycles.Even at a higher current density up to 100 C,there is still a capacity of 104 mA h g-1.(2)Oxygen doped carbon nanotube frameworks(a-CNTs)were prepared by pyrolysis and used as the substrate for sodium deposition.Both theoretical calculations and experimental results prove that the surface oxygen containing groups of a-CNTs have stronger binding energy than carbon nanotubes with sodium,which can greatly increase the "sodiophilic" of the electrode surface,which is beneficial to the homogeneous nucleation and uniform deposition of Na metal.The a-CNTs half cell can stably cycle over 1600 cycles with the coulombic efficiency of 99.9%at 1 mA cm-2 with an area capacity of 1 mA h cm-2.When the area capacity is increased to 8 mA h cm-2,the a-CNTs/Na symmetric cell can run for 100 cycles at a current density of 5 mA cm-2.(3)Nitrogen and oxygen co-doped carbon nanofiber network coated carbon felt(NOCF)was prepared by the self-assembly process of polyaniline and used as the substrate for sodium deposition.The nano-network structure on the surface of NOCF can effectively disperse the sodium ions flux inside the electrode and ease the formation of sodium dendrites.The abundant "sodiophilic" N-and O-containing functional groups on the surface of NOCF can significantly improve the unniform nucleation and stable deposition of sodium metal.The NOCF/Na symmetric cell can maintain a stable overpotential lower than 20 mV even after 600 h cycles at 1 mA cm-2 with an areal capacity of 1 mA h cm-2,and the NOCF/Na presents a flatter voltage plateaus compared with CF/Na symmetric cells.